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Avoid Common Filing Mistakes for Business and Withholding Income Tax

Filing electronically is the best way to avoid common, and sometimes costly, mistakes. IL-1120, IL-1120-ST, and IL-1065 filers can file electronically using approved Tax-Prep software or through a preparer. IL-990-T and IL-1041 filers can file free using MyTax Illinois. Once you submit your return electronically, do not mail a copy. Use the following information to assist you in avoiding processing delays.

General information for avoiding common mistakes

For Illinois Withholding Income Tax filers

Apportioning business income and allocating other income has specific formulas

Attach all necessary supporting schedules or forms to your return

File according to how you file with the IRS

Claim payments and carry forward amounts correctly

For unitary filers

If you need to correct or change your original return after it has been filed, be sure to use the corresponding amended return not another original return. (For example, if you submitted your original IL-1120 and need to correct or change the amounts reported, use Form IL-1120-X).